1. Field of the Invention
This invention pertains to a process for forming a photosensitive element having particulate material on a layer of photopolymerizable material. The invention also relates to an apparatus for applying the particulate material onto the photopolymerizable layer. The particulate material may be applied to the photosensitive element that is in sheet form, a web or a cylindrically-shaped form. In particular, the method and the apparatus apply particulate material onto the photopolymerizable layer of a cylindrically-shaped photosensitive printing element suitable for use in flexographic printing.
2. Description of Related Art
Photopolymerizable material can be formed into sheets or layers by several known methods such as solvent casting, hot pressing, calendering and extrusion. The photopolymerizable material generally comprises an elastomeric binder, at least one monomer and a photoinitiator. A preferred method of forming photopolymerizable material for use in a flexographic printing element is by extrusion calendering the photopolymerizable material. In extrusion calendering, the printing element is prepared by passing a mass of hot photopolymerizable material into an extrusion die that forms a layer, passing the layer into the nip of a calender and, while still hot, calendering the photopolymerizable material between two flat surfaces, generally two flexible films, to form a multilayer web. The printing element as a multilayer web can be cut into suitable size sheets. Extrusion and calendering of photopolymeric compositions are disclosed, for example, in Gruetzmacher et al., U.S. Pat. No. 4,427,759; and in Min, U.S. Pat. No. 4,622,088.
Although a photosensitive printing element is typically used in sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form. A continuous printing element has applications in the flexographic printing of continuous designs used in wallpaper, decoration and gift wrapping paper, and tight-fit conditions for registration, since the designs can be easily printed without print-through of the plate seam. Furthermore, such a continuous printing element is well-suited for mounting on laser exposure equipment where it can replace the drum or be mounted on the drum for exposure by a laser to achieve precise registration.
The formation of a seamless, continuous printing element can be accomplished by several methods. The photopolymerizable flat sheet element can be reprocessed by wrapping the element around a cylindrical form, usually a printing sleeve or the printing cylinder itself, and fusing or joining the edges together to form a seamless, continuous element. Processes for joining the edges of a plate into a cylindrical form have been disclosed, for example, in German patent DE 28 44 426, United Kingdom patent GB 1 579 817, European patent application EP 0 469 375, U.S. Pat. No. 4,883,742, and U.S. Pat. No. 4,871,650. A problem with the prior methods of joining the edges to form the continuous cylinder is that the results of printing with the joined edge element are often unsatisfactory, particularly when the joined edge falls within the effective printing area of the plate. Although the prior art often describes the so-formed continuous element as “seamless”, the joined seam has not completely formed a continuum of the photopolymerizable layer at the edges since the joined seam is visible in and interrupts the printed image.
U.S. Pat. Nos. 5,798,019 and 5,916,403 disclose an apparatus and a process for forming a cylindrical photosensitive element of uniform thickness on a flexible sleeve without sanding, grinding or additional polishing steps. The method involves supplying a stream of molten photopolymerizable material onto the sleeve supported directly on a mandrel, calendering the molten photopolymerizable material to have a substantially constant thickness on the sleeve, moving the sleeve around and along the mandrel in a helical fashion to polish an outer surface of the element, and during the calendering step, heating the photopolymerizable material.
A flexographic printing form is made from a sheet or cylindrical photosensitive element by imagewise exposing the element to actinic radiation to selectively polymerize the photopolymerizable layer, and treating the element with a suitable solvent or heat to remove the unexposed areas of the photopolymerizable layer, thereby leaving a printing relief which can be used for printing.
Imagewise exposure of a photosensitive element requires the use of a mask having transparent and opaque areas covering the photopolymerizable layer. The mask may be a phototool that is a photographic negative of the desired printing image. In some applications, it is desirable to eliminate the phototool by directly recording information on the photosensitive element, e.g., by means of a laser beam. In particular, digitized imaging without a phototool is well-suited for making a seamless, continuous printing form. The image to be developed could be translated into digital information, and the digital information used to place the laser for imaging to form an in-situ mask on the photopolymerizable layer.
U.S. Pat. Nos. 5,888,697 and 5,888,701 disclose a photosensitive printing element having an overall layer of powder material and a process for making a flexographic printing form from such an element. The photosensitive element includes a support, a photopolymerizable layer, and a layer of powder material on the photopolymerizable layer. The is powder material may be opaque or transparent depending upon desired use. Typically the surface of the photopolymerizable layer is inherently tacky due to the elastomeric binder and/or migratory compounds such as monomer and plasticizer present in the photopolymerizable layer. Due to the tackiness of the photopolymerizable layer, the powder material adheres to the exterior surface of the photopolymerizable layer, preferably to form an overall layer of particulate material on the element. The particulate layer can function as a release layer, a masking layer such as an infrared-sensitive layer for forming an in-situ mask, or can function to alter the surface characteristics of the photopolymerizable layer.
Apparatuses for automatically applying particulate material, such as toner, onto a tacky surface are known. The tacky surface is provided by a sheet-like photosensitive element, typically a pre-press proof form, which has been imagewise exposed to form nontacky areas and tacky areas on the surface. Examples of automatic toning apparatuses are disclosed by Cohen et al. in U.S. Pat. No. 3,980,047; Tobias in U.S. Pat. Nos. 4,069,791 and 4,425,867; and Martin in U.S. Pat. No. 4,414,916. Generally, the apparatus includes a hopper for supplying and dispensing the toner onto an applicator that transfers the toner to the tacky surface. The applicator typically is a brush pad that can oscillate laterally of the direction of movement of the tacky surface. Martin in U.S. Pat. No. 4,414,916 also discloses that the photosensitive element can be heated prior to toning to enhance the tackiness of the exposed latent image on the surface. Although the use of automatic toning apparatuses generally improve the coverage of the toner on the tacky surface over hand application of the toner, nonuniformity of the toner layer, observed as bands or lanes of high density areas and/or low density area, may occur along the direction of movement of the element.
A problem associated with applying particulate material to the photopolymerizable layer is that it may take multiple applications of particulate material to thoroughly cover to a substantially uniform state the exterior surface of the photopolymerizable layer. Generally, it is desirable to have the particulate material as an overall layer so that the surface tackiness of the photopolymerizable layer is reduced or eliminated. If the particulate material is transparent, such an overall layer may function as the release layer. If the particulate material is opaque and is used to form an in-situ mask, overall coverage of the element is necessary to attain the density of the layer of particulate on the photopolymerizable layer in order is for the opaque layer to block actinic radiation to the photopolymerizable layer. In addition to multiple applications, each application of particulate material may take extended periods of time to adhere the particulate to the tacky surface.
Another problem associated with applying particulate material to the photopolymerizable layer is that because the photopolymerizable layer is so inherently tacky, air borne contaminates may first adhere to the surface of the photopolymerizable layer prior to the application of the particulate material. As such, the particulate material does not adhere to the photopolymerizable layer where contaminates are located and a non-uniform layer of particulate material results. The non-uniformity caused by contaminates creates pinholes in the layer. Pinholes are problematic particularly when the particulate layer is opaque and blocks the underlying photopolymerizable layer from actinic radiation during imagewise exposure. Pinholes allow radiation to expose the underlying photopolymerizable layer. This pinhole exposure causes print through and undesirable specks on the floor of the processed printing element which result in background printing on press.
Thus, the problem is to achieve the desired quality and density of the layer of particulate material on the photopolymerizable layer while maintaining productivity and low cost of the process to prepare a photosensitive printing element.